1. Technical Field
The present invention relates to a transmitter optical subassembly and a transmitter optical module installing the same.
2. Related Prior Art
In general, it is known, as configurations to connect a device for switching a current supplied from a current source among the semiconductor laser diode (LD) driver systems, a series configuration where switching devices are inserted in series to a light-emitting device and a shunt configuration where switching devices are inserted in parallel to a light-emitting device. A Japanese Patent application published as JP-2001-015854A discloses a typical example of series configuration. The shunt configuration has advantageous in that it uses a smaller number of devices than the series configuration and allows high-speed operation, while the shunt configuration has a drawback that the degree of modulation permitted is small and the controllability of a current is poor.
A Japanese Patent application published as JP-2004-179591A discloses another LD driver. This driver reflects the switching status of the reference current, which flows in the reference current path of the current mirror circuit, by toggling the switch connected to the reference current path, to the current supplied to the LD connected to the other current path of the current mirror circuit. In order to make steep the leading edge and the falling edge of the mirrored current supplied to the LD, a circuit is added that generates a pulse current synchronous with the leading edge and the falling edge of a signal to select the switching state. With this additional circuit, the LD driver increases/decreases the transient currents on the edges of the signal to perform high-speed LD switching.
Optical communications in recent years reaches a transmission speed exceeding 10 Gbps. In the high-speed transmission range over 10 Gbps, the frequency response of the wiring connecting a light-emitting device and a driver, that is, the transmission impedance becomes important. Especially, a waveform may be degraded by the parasitic inductance of a lead pin in a transmitter optical subassembly. Or, the leading edge or the falling edge of a signal may be delayed by a parasitic capacitance.
To solve these problems, a package with good controllability of the transmission impedance, for example, a butterfly package is used as a package for a transmitter optical subassembly. However, the butterfly package is expensive and is not effective in terms of high-speed transmission when the light emitting device is directly driven. For indirect modulation system that uses a MZ (Mach-Zender modulator) or an EA modulator (Electro-absorption modulator), the termination of a signal line may be easily and reliably carried out. For a system for directly modulating a light-emitting device, the internal resistance of the light-emitting device becomes 3 to 30 ohms in the case of a laser diode (LD). When a resistor of 1 to 40 ohms is serially inserted to the LD to terminate the transmission line, the inserted resistor and the junction capacitance of the light-emitting device constitute an integrator, which causes a problem in the high-speed transmission.
It is known, what is called, a CAN-type package as another form for the optical subassembly. The CAN-type package is less expensive than the butterfly package. A problem with the CAN-type package is that impedance matching of a lead pin is hard to ensure and the high-speed modulation is difficult. Another problem is the heat dissipation characteristic of a driver when a driver is installed within the CAN-type package. The CAN-type package has less tolerance freedom in terms of its shape and specifications including its diameter, the number of pins and LD installation method than the butterfly package. When a driver is installed within the CAN-type package, it is difficult to efficiently dissipate the heat of the driver out of the package. Large quantity of heat generated in a transmitter optical subassembly increases the temperature of the LD, which degrades the light emission characteristic and the reliability of the optical assembly.
The invention relates to a transmitter optical subassembly that may improve a high-frequency performance without increasing the power consumption of the transmitter optical subassembly and an optical data link installing the same.